\section{Electronics Design} % (fold)
\label{sec:Electronics Design}

The electronics subsystem comprises the main flight computer, actuator driver boards, power supply and the inertial measurement unit.

\subsection{Flight Computer} % (fold)
\label{sub:Flight Computer}

The flight computer was specified with the help of Fergus Noble, a CUSF teammate, and the requirements list is can be seen in Appendix \ref{sec:Requirements for Avionics}. The collaboration ensures that the vehicles flight computer might be flexible enough to form the basis of several future projects, as well as meeting the specification required for this project. 

\subsubsection{Processor Selection} % (fold)
\label{ssub:Processor Selection}

The onboard processor for the vehicle is one of the most important components - it is the heart of the digital control system, and the capabilities of the controller depends on large extent on the computational power of the processor. If during development it was discovered to lack a necessary capability, it would likely leave to costly and time consuming changes to the hardware and software.

The processor selected was the Phillips/NXP LPC2368 - an ARM7-TDMI processing core with a variety of peripherals \cite{lpc}. The device is low cost and readily available from convenient outlets such as Farnell \cite{farnell} in single quantities. Its arithmetic logic unit can perform 60M instructions per second, and whilst being fixed point, this is plenty fast enough to perform the estimation and control with software floating point libraries. It also has a hardware SD card interface, which allows data to be written to the SD card at much higher rates than the legacy SPI interface. There was a strong desire to stay with the ARM7 core architecture instead of ARM9 because of the large jump in programming complexity that usually accompanies the latter.

\subsection{Flight computer board design} % (fold)
\label{sub:Flight computer board design}

The flight computer printed circuit board (PCB) lay-out was done by the author using CadSoft Eagle Schematic and Layout software \cite{eagle}. The PCB is a 4-layer board of dimensions 110mm x 50mm and can be seen in Figure \ref{fig:badger2}. The PCB footprint and schematics can be found in Appendix \ref{sec:layout}. The blank PCB fabricated by Newbury Electronics \cite{newbelec}.

\begin{figure}
	\centering
	\includegraphics[width=\textwidth]{hobble/badger2annotated.jpg}
	\caption{The Finished Flight Computer}
	\label{fig:badger2}
\end{figure}

The layout can be found in Appendix . The flight computer board is a mixed signal environment, and as such a lot of care was taken in the design of the grounding - a `star' grounding topology was used to minimise interference between enviroments with RF, high speed digital switching, and sensitive analogue sensors. The two inner layers are 0V and 3.3V, ground and power respectively, with the signal layers on the surfaces. Each of the signal layers has a `ground pour' between tracks to further minimise the ground impedance. As the author's first four-layer board, and with it being relatively compact and with a large number of components, and the cost of getting such a board manufactured (the single largest expense of the project), a great deal of time (approximately 80-100 hours) was spent on the layout. Despite this, some errors were made, such as can be seen to the left of the `X,Y,Z Gyroscopes' where the author designed an oscillator footprint to be a mirror image of the correct footprint. This mistake was fixed with the liberal use of green `bodge-wires'.

The PCB is composed almost entirely of fine-pitch surface mount components. These were soldered in the CUSF lab with solder paste and a cheap infra-red `toaster oven' into which the PCB, solder paste and components were placed and heated to 250\textdegree C until the solder paste reflowed. The PCB was then left to cool.

% subsection Flight computer design (end)

\subsection{Power supply} % (fold)
\label{ssub:Power supply}

\subsubsection{Power Architecture} % (fold)
\label{ssub:Power Architechture}


The vehicle must maintain two power buses for operation - a 5V `clean' bus for the electronics and a 24V bus for the electronic motors used for actuation. 2 redundant DC/DC switching converters were used to provide a 5V bus for efficiency. On each board powered by this 5V bus, linear step-down regulators were used to convert to lower voltages as required (for example 3.3V for the main microcontroller). Whilst linear converters are often rejected on the basis of inefficiency (the voltage that gets dropped across them is dissipated as heat) this extra heat was considered an advantage in this context, so as to provide extra warmth in the electronics bay.

The 24V bus comes directly from the batteries, and as such is subject to variations in voltage according to battery condition and load. Feedback controllers on the motors accomodate this variation.

\subsubsection{Batteries} % (fold)
\label{ssub:Batteries}

Several battery chemistries were considered with the following criteria:
\begin{itemize}
	\item Must be rechargeable (to reduce cost)
	\item Low Temperature Performance
	\item Energy Density
\end{itemize}

Lithium Polymer batteries were the chosen chemistry, specifically Kokam 50106100 cells. These have a capacity of 5Ah and a nominal voltage of 3.7V, so 6 were used in series, giving a total battery pack mass of 690g.
% subsubsection Batteries (end)

% subsubsection Power supply (end)

% subsubsection Processor Selection (end)

% subsection Flight Computer (end)
% section Electronics Design (end)
